1. Field of the Invention
The present invention relates to a low reflection limiter, which is to be typically used for an active phased array antenna and also relates to a transmitting/receiving module utilizing such a limiter.
2. Description of the Related Art
A transmitting/receiving module, which includes amplifiers for amplifying a transmitting signal and a receiving signal, is typically used for an active phased array antenna. The active phased array antenna has a plurality of radiators (i.e., antennas) for radiating the transmitting signal into the air. In a typical example, each of the radiators is provided with a transmitting/receiving module. In addition, there is provided, between the radiator and the transmitting/receiving module, a transmitting/receiving separation circuit for achieving separation between the transmitting signal and the receiving signal. Japanese Laid-Open (KOKAI) Publication No. H09-270601 describes a circuit for connecting such a transmitting/receiving module to the transmitting/receiving separation circuit.
Now, the conventional transmitting/receiving module will be described hereunder with reference to FIG. 5.
The transmitting signal is inputted from an input terminal “IN” into the transmitting/receiving module 50. A phase shifter 51 turns the phase of the transmitting signal so that the transmitting signal has a desired phase. Then, the transmitting signal is supplied to a power amplifier 53 through a transmitting/receiving switch 52, which is flipped to a transmitting side Sa, to provide an amplified signal. Such a transmitting signal passes through a transmitting/receiving separation-non-reciprocal circuit element, e.g., a circulator 54 in a direction of an arrow “Y” as shown in FIG. 5 in a low-loss manner. Then, the transmitting signal is sent from an output terminal “OUT” to a radiator 55 and radiated from the radiator 55 into the air. The transmitting signal, which has been radiated into the air and then reflected from a target, is then received by the radiator 55. The thus received signal is sent from the circulator 54 to a low-noise amplifier 56 to ensure low noise. Then, the signal passes through the transmitting/receiving switch 52, which is flipped to a receiving side Sb, the phase shifter 51 and the input terminal “IN”, and is then sent to a signal processing circuit (not shown).
The active phased array antenna, which is provided with a plurality of combinations of the above-described transmitting/receiving module 50 and radiator 55, causes the transmitting signals, which have been radiated from the respective radiators 55 to provide a desired radiation pattern.
When the transmitting signal radiated into the air is reflected from an obstruction, existing at a short distance away from the radiator 55, the radiator 55 receives the reflected signal at a large intensity. Alternatively, radiating the signal in a certain direction may cause the radiator to receive the signal at a large intensity due to interference with the adjacent radiator. The above-described reflection from the obstruction or the interference with the adjacent radiator may cause undesired transmitting signal parts of components (hereinafter referred to as the “undesired reflection parts or components”) to be inputted to the radiator. The undesired reflection components have various power amplitudes and phases. Such an undesired reflection component may have the power amplitude, which reaches up to 105 (hundred thousands) times as large as a regular receiving signal.
Inputting the undesired reflection component having the large power amplitude into the transmitting/receiving module without applying any processing to the signal may cause damage to the low-noise amplifier for the receiving signal or a breakage thereof. When the undesired reflection component is inputted to the output side of the power amplifier for the transmitting signal for some reason, there apparently arises a situation, that is equivalent to application of load having adverse reflection properties to the power amplifier, thus disabling the desired performance from being achieved.
In view of these defects or problems, the conventional transmitting/receiving module is provided, for example, in the receiving system, to which the receiving signal is to be transmitted, with a protection circuit to protect the low-noise amplifier for the receiving signal and the power amplifier for the transmitting signal from the undesired reflection component.
Now, description will be given below of the conventional transmitting/receiving module provided with the protection circuit, with reference to FIG. 6. The same references as those in FIG. 5 are assigned to the corresponding components as shown in FIG. 6. Description of the same components will therefore be omitted.
In this conventional example, the receiving system is provided with a switch 61 serving as the protection circuit. The switch 61 is connected to a terminating resistor (which may be called a termination resistance) 62. The switching operation of the above-mentioned switch 61 is carried out in synchronization with the transmitting/receiving switch 52. The switch 61 is flipped to the terminal Sa when transmitting a transmitting pulse signal, and to the other terminal Sb when receiving such a signal.
According to the above-described configuration, the switch 61 interrupts a signal path of the receiving system, when transmitting the transmitting pulse signal. As a result, the undesired reflection components due to reflection of signals from an obstruction, existing at a short distance away from the radiator, or interference with the adjacent radiator, are prevented from being inputted to the low-noise amplifier 56, thus providing protection of the low-noise amplifier 56. In addition, the undesired reflection components are converted into heat by means of the terminating resistor 62. It is therefore possible to prevent the undesired reflection components from being inputted to the output side of the power amplifier 53, thus providing protection of the power amplifier 53.
Another example of the conventional transmitting/receiving module provided with the protection circuit will be described with reference to FIG. 7. The same references as those in FIG. 5 are assigned to the corresponding components as shown in FIG. 7. Description of the same components will therefore be omitted.
In this conventional example, a four-port circulator 71 is applied as a non-reciprocal circuit element for transmitting/receiving separation. The four-port circulator 71 is composed of, for example, a pair of three-port circulators, i.e., the first and second circulators 71a, 71b. The first three-port circulator 71a is connected at one terminal thereof with one terminal of the second three-port circulator 71b. In addition, a terminating resistor 72 is connected to the other terminal of the second three-port circulator 71b. A limiter diode 73 is connected between the four-port circulator 71 and the low-noise amplifier 56.
In this case, the undesired reflection component passes through the four-port circulator 71 and the power amplitude of the undesired reflection component is then limited by means of the limiter diode 73. It is therefore possible to decrease the power amplitude of the undesired reflection component, which has leaked from the limiter diode 73 to the low-noise amplifier 56, thus providing protection of the low-noise amplifier 56. In addition, almost all the remaining undesired reflection components, which have not leaked from the limiter diode 73 to the low-noise amplifier 56, are reflected by the limiter diode 73. The thus reflected undesired reflection components are supplied to the terminating resistor 72, which is connected to the second three-port circulator 71b, and then converted into heat. It is therefore possible to prevent the undesired reflection component from being inputted to the output side of the power amplifier 53, thus providing protection of the power amplifier 53.
A further example of the conventional transmitting/receiving module provided with the protection circuit will be described with reference to FIG. 8. The same references as those in FIG. 5 are assigned to the corresponding components as shown in FIG. 8. Description of the same components will therefore be omitted.
In this conventional example, a switch 81 is substituted for the non-reciprocal circuit element to a position at which there is achieved separation between the transmitting signal and the receiving signal. The switching operation of the above-mentioned switch 81 is carried out in synchronization with the transmitting/receiving switch 52. The switch 81 is flipped to the terminal Sa when transmitting a transmitting pulse signal and to the other terminal Sb when receiving such a signal.
According to the above-described configuration, the switch 81 interrupts a signal path of the receiving system, when transmitting the transmitting pulse signal, thus preventing the undesired reflection component from being inputted to the low-noise amplifier 56. In such measures, the undesired reflection component is inputted to the output side of the power amplifier 53, thus disabling the desired performance from being achieved.
In the configuration as shown in FIG. 6, a PIN-diode or a field-effect transistor (hereinafter referred to as the “FET”) is normally used as the switch 61.
The PIN-diode, which has a low insertion loss, is useful to manufacture the transmitting/receiving module having a low noise factor and good noise characteristics. A driving circuit for turning the PIN-diode “ON” or “OFF” at high speed is, however, required, thus leading to an increased cost and a large size. In addition, power consumption required for driving the PIN-diode becomes measurable. In addition, it is difficult to prepare the switch with the PIN-diode in the form of MMIC (Monolithic Microwave Integrated Circuit), thus leading to a few cases where the switch utilizing the PIN-diode is used as the MMIC.
The FET has an advantage that the power consumption required therefor is smaller than the PIN-diode. In addition, a control circuit for turning the FET “ON” or “OFF” at high speed may be composed of a CMOS (complementary metal oxide semiconductor) logic circuit, thus facilitating supply of the device at low cost. The above-mentioned control circuit for turning the FET “ON” or “OFF” is however required, leading to a large size. The FET has a high insertion loss, with the result that increase in power amplitude of the undesired reflection part or component deteriorates the noise characteristics of the receiving system.
The measures according to the structure of FIG. 7 have advantages that neither driving circuit nor control circuit is required, the noise characteristics are good and the power consumption is low. However, in this example, the use of two circulators causes the increased cost, the enlarged size and the increased mass. It is therefore difficult to provide a small-sized or low-cost transmitting/receiving module.
The measures of the structure of FIG. 8 provide the good noise characteristics. The undesired reflection component is, however, inputted to the power amplifier for amplifying the transmitting signal. As a result, there cannot be provided one of the fundamental functions of the protection circuit, e.g., the function of giving stability to the performance of the power amplifier, which is located on the transmitting side. Accordingly, the measures as shown in FIG. 8 are limitedly applied to a case where the undesired reflection component is small, for example, a beam scanning angle is narrow. The cost of the device, the power consumption and the size thereof are substantially equal to those as shown in FIG. 6. The further reduction in size is desired.